Problem executing a Java Project class in GXP project

Hi, I created a class in a java project that uses the Runtime class and it works well within this project. When I import the jar of the java project into the Gxt project external library and launch the execution, an error message appears say:No source code found for Runtime class.
Secondly, I decided to create the class directly in the Gxt project while stile using the same Runtime class. Upon execution, thesame error is shown.
So please can someone help me solve this, or give another method which we can use to execute runtime class methods.